1. Field of the Invention
The present invention relates to an aperture diaphragm which is arranged in an optical system, the optical system, and an exposure apparatus. The present invention is preferably usable for an optical system of an exposure apparatus which is used in the lithography step of producing a device including a liquid crystal display element, a semiconductor element, etc. The present invention also relates to a method for producing an electronic device using the exposure apparatus.
2. Description of the Related Art
In recent years, an exposure technique, which is using the EUV (Extreme Ultra-Violet light, Extreme Ultra-Violet light beam) having a shorter wavelength (about 11 to 14 nm), instead of the conventional ultraviolet light, has been developed in order to improve the resolving power of the optical system restricted by the diffraction limit of the light, as the semiconductor integrated circuit becomes fine and minute. Accordingly, it is expected that the exposure can be performed with a pattern size of about 22 to 45 nm. The refractive index of a substance is close to 1 in the wavelength region of the EUV light. Therefore, it is impossible to use the transmission dioptric optical element such as the conventional lens; and the mirror, which is an optical element of the reflection type, is used.
An exposure apparatus (hereinafter referred to as “EUV exposure apparatus”), which uses the EUV light as the exposure light (exposure light beam), is constructed to include a light source which generates the EUV light; a group of illumination optical system mirrors which shape the light beam (light); a reflection type mask; a group of projection optical system mirrors which reduce and project a circuit pattern on the mask onto a wafer; and the like. Those usable as the light source include those of the laser plasma system, the discharge plasma system, etc. It is necessary that the optical path for the exposure light should be in vacuum in order to suppress the absorption of the light. The groups of mirrors, which constitute the optical systems, are accommodated in a vacuum chamber. Such an EUV exposure apparatus is described, for example, in International Publication No. 2007/004358.
In the conventional exposure apparatus which uses the ultraviolet light as the exposure light, the projection optical system thereof is the transmission dioptric system, which is the rotationally symmetrical optical system about the center of the optical axis. Therefore, all of the optical elements including the pupil (or the aperture diaphragm) are rotationally symmetrical. The aperture diaphragm necessarily has the optimum shape which is circular and which is parallel to the reticle surface and the wafer surface. Such a circular aperture diaphragm has been hitherto used.
On the other hand, in those using the catoptric optical system as the projection optical system such as the EUV exposure apparatus described in International Publication No. 2007/004358, a field which is separated (away) from the optical axis is used in order to avoid the light flux interference. However, an aperture diaphragm, which is circular or elliptical, is arranged in parallel to the reticle surface and the wafer surface at a position at which the light flux is allowed to pass obliquely with respect to the optical axis.
However, such an aperture diaphragm is not provided at an optimum position in relation to respective orientations in the field. For this reason, it is feared that the deterioration of the optical performance, which includes the distortion of the image, etc. might be caused when the image is projected onto the wafer.